Five barrier removals in the Danube River, Slovakia

Introduction

The Danube River, Europe’s second-largest river, flows through ten countries, supplying drinking water to 20 million people and connecting 83 million within its drainage basin. In Slovakia, the river transforms from a fast-flowing mountain stream to a slow-moving lowland river, depositing thick layers of sediment. This process has created the unique Danube inland delta, particularly in the region between Bratislava and Klížská Nemá. This area, with its main riverbed, side arms, islands, swamps, and meadows, was once rich and diverse, supporting various animal species.

However, 19th and 20th-century alterations, including the construction of the Gabčíkovo hydropower plant and artificial navigation channels, diverted water from the original riverbed. This significantly impacted the branch systems between Dobrohošť and Sap village, causing degradation over the decades. Despite these interventions, the area remains a vital ecosystem with a diverse wetland complex spanning over 4500 hectares, supporting a high degree of biodiversity.

Project context and opportunity

Widespread infrastructure development along the Danube River has led to deteriorating water quality, reduced river dynamics, and significant biodiversity loss. The construction of hydroelectric power plants and navigation channels altered the river’s flow, converting many flowing sections into stagnant ones. Since 1992, up to 50% of Slovakia’s Danube habitats have been lost, particularly impacting the inland delta between Dobrohošť and Sap villages. Fish populations decreased by up to 70%, mainly due to inaccessible or degraded spawning grounds and lost dynamics.

The project’s primary objective was to restore a sidearm stream, “Foki,” by reconnecting it to the main riverbed, reviving the natural water regime and dynamics. This restoration would have gradual and far-reaching effects, benefiting Danube habitats, including breeding and feeding areas for endangered species. It would also enhance groundwater levels, water retention, and flood protection while aiding native fish populations and alluvial forests.

Project aims

This project aimed at restoring a section of the Danube by reconnecting the degraded Foki sidearm to the Danube, removing four artificial barriers such as traverses and groynes. Removing the structures would allow year-round access to the mainstem, a unique feature in Slovakia. This project aligns with the Ramsar Convention, Water Framework Directive, Slovak Water Strategy, and EU Biodiversity Strategy 2020 by implementing conservation measures outlined in existing management plans, emphasising the importance of preserving this vital ecosystem.

Project achievements

The removal of five barriers from the Danube side arm – three old groynes and two traverses – was the first step in the complex side arm restoration. This was followed by dredging of the riverbed and the reconnection of the Foki side arm to the Danube riverbed. It was the first time in Slovakia that old groynes were modified, and also the first time a side arm restoration was carried out within the Old Danube – a river section highly negatively affected by the construction of the Gabčíkovo waterworks.

After several decades, Danube fish species can once again use their original spawning grounds in the Foki side arm, along with many other species that have regained suitable conditions and appropriate feeding or breeding habitats.

The impact of the restoration works will be monitored over the next several years, with a focus on hydrological and ichthyological observations.